Communication system can be classified as either multi-point or point-to-point. In the latter, any given transmitter/receiver unit can only communicate with one other transmitter/receiver unit, while in the former, a central transmitter/receiver unit can communicate with a plurality of other transmitter/receiver units. Communication systems can also be differentiated based on the information flow. In unidirectional systems, the information only flows from a transmitter at one location to a receiver at another location. Alarm or monitoring systems are examples of such unidirectional systems. Bidirectional communications systems, on the other hand, send information back and forth between two or more locations at communication rates which may or may not be the same in each direction. Regardless of such classifications, there are a myriad of different coding schemes used to prepare the information signal for transmission through a medium. To increase the probability of accurately recovering the information signal in a receiver unit, many coding schemes utilize distortion compensation apparatus and/or error correction.
Equalizers are one class of devices used in communications systems to compensate for the distortion in a communications channel. Equalizers fall into two broad categories: fixed and adjustable. In a fixed equalizer, the average electrical characteristics of the communications channel are determined and a fixed amount of equalization is then designed into the equalizer which compensates for the distortion characteristics of an average channel. In an adjustable equalizer, the channel is monitored and the equalization provided in the equalizer is varied so as to provide that necessary to match the distortion characteristic present at the time of monitoring. Such monitoring may be done when customer-controlled information symbols are transmitted and the equalizer is continually adjusted after one or more symbols are received. In such case, the adjustable equalizer is called an adaptive equalizer. If the adjustment of the equalizer is provided in response to the transmission of a known symbol sequence, called a training sequence, which is transmitted at predetermined, substantially separated time intervals, the adjustable equalizer is called an automatic equalizer.
Error correction can be provided by either block or convolutional coding. In block coding, one or more error correction bits are transmitted along with a "block" of one or more information bits. Each of these error correction bits has a value which is determined by the value of the information bit or bits in the associated block. This process of transmitting error correction bits is also utilized in convolutional coding but unlike block coding, the value of each error correction bit in convolutional coding is a function of the information bits in the associated block and some predetermined number of previously transmitted blocks.
Coding gain is a term which refers to the increased performance of a system resulting from the use of error correction. It is defends as the amount by which the signal-to-noise ratio may deteriorate for a system utilizing error correction before the bit error rate for this system equals that of the same system without error correction. This term can be calculated analytically for any system and for purposes herein the resulting analytically derived quantity is referred to as the theoretical coding gain.
In actual systems using error correction, it has long been observed that there is a substantial difference between the theoretical coding gain and that determined from actual measurements. For example, in some voiceband transmission systems, the theoretical coding gain may be 3 to 4 dB while the measured coding gain is only 1 to 2 dB. This difference can degrade performance below systems objectives and is especially troublesome is high-speed transmission systems. Prior art techniques to correct this shortcoming have focused on the use of apparatus in the transmitter which reorders the sequence of symbols that would ordinarily be transmitted. In the receiver, apparatus is also added which restores the original symbol sequence. While this technique has provided satisfactory results in certain applications, it is unsuited for many others as it adds considerable delay to the signal transmission and, of course, adds considerable circuit complexity to both the receiver and transmitter. Moreover, the added delay and circuitry complexity becomes progressively worse with increasing levels of distortion. As a result, the utility of the prior art technique is severely circumscribed. Therefore, it would be extremely desirable if a technique could be developed which permits the theoretical coding gain associated with error correction to be realizable in an actual system regardless of the degree of channel distortion. It would be especially beneficial if this could be provided without adding significant complexity to either the transmitter or receiver circuitry.